1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of fabricating a liquid crystal display device by using a liquid crystal dropping scheme.
2. Discussion of the Related Art
There are ultra-thin flat panel displays having a display screen of a few centimeters thick operated at a low voltage. Such displays consume less power and are portable. Among them, a liquid crystal display device has been most widely used in various fields such as a notebook computer, a monitor, a spacecraft, an airplane, and the like.
Such a liquid crystal display device generally includes a lower substrate having a plurality of thin film transistors and pixel electrodes formed thereon, an upper substrate having a black matrix layer, a color filter layer, and a common electrode formed thereon so as to face into the lower substrate, and a liquid crystal layer between the lower and upper substrates. An electric field is generated between the substrates by a voltage applied to the pixel and common electrodes so as to drive the liquid crystal layer. And, light transmittance is controlled through the driven liquid crystal layer so as to display images on the display screen.
In the above-described liquid crystal display device, the liquid crystal layer is formed between the lower and upper substrates by a vacuum injection method using a capillary phenomenon and a pressure difference. A method for fabricating a liquid crystal display device according to a related art using the vacuum injection method is explained as follows.
A plurality of thin film transistors and pixel electrodes are formed on a lower substrate. On an upper substrate, a black matrix layer, a color filter layer, and a common electrode are formed thereon.
A plurality of spacers are formed on one of the upper and lower substrates so as to maintain a uniform cell gap between the upper and lower substrates. A sealant is then formed on the circumference of one of the upper and lower substrates so as to prevent liquid crystals from leaking outside as well as bond the upper and lower substrates to each other. In this case, a thermo-hardening sealant such as an epoxy sealant is widely used for the sealant.
And, the upper and lower substrates are bonded to each other. The epoxy sealant, a mixture of epoxy resin and initiator, is heated for polymerization through cross-linkage so as to work as a sealant having an excellent adhesion.
The bonded substrates are loaded on a vacuum chamber to maintain a vacuum condition inside the bonded substrates, and then dipped into the liquid crystal. Once the vacuum condition is achieved inside the bonded substrates, the liquid crystal is sucked into the space between the bonded substrates by a capillary phenomenon.
When the bonded substrates are filled with a predetermined amount of the liquid crystal, a nitrogen gas (N2) is slowly injected into the vacuum chamber so as to cause a pressure difference between the space of the bonded substrates and the ambience. Hence, the bonded substrates are completely filled with the liquid crystal so that a liquid crystal layer is formed between the upper and lower substrates.
Yet, such vacuum injection requires a long process time as a display screen becomes larger, thereby reducing productivity.
In order to overcome such a problem, a liquid crystal dropping method is newly proposed. A method of fabricating a liquid crystal display device using a liquid crystal dropping method according to a related art is explained as follows.
FIGS. 1A to 1D are perspective views illustrating a method of fabricating a liquid crystal display device using a liquid crystal dropping method according to a related art.
Referring to FIG. 1A, lower and upper substrates 1 and 3 are prepared for a liquid crystal dropping method. A plurality of gate and data lines are formed on the lower substrate 1 to cross with each other defining pixel areas. A thin film transistor is formed on each of the intersections between the gate and data lines. A pixel electrode is formed at each of the pixel areas so as to be connected to the thin film transistor.
A black matrix layer is formed on the upper substrate 3 to prevent light from leaking from the areas where the gate lines, the data lines, and the thin film transistors are formed. A color filter layer of red, green, and blue is formed on the upper substrate 3. A common electrode is formed on the upper substrate 3. An alignment layer is formed on at least one of the lower and upper substrates 1 and 3 for an initial alignment of the liquid crystal.
Referring to FIG. 1B, a sealant 7 is formed on the lower substrate 1. A plurality of liquid crystal droplets 5 are dispensed on the lower substrate 1 for forming a liquid crystal layer. And, spacers are scattered on the upper substrate 3 so as to maintain a uniform cell gap.
Referring to FIG. 1C, the lower and upper substrates 1 and 3 are attached to each other.
In the conventional vacuum injection method for fabricating an LCD, two substrates have to be attached to each other before the liquid crystal is injected therebetween. In the liquid crystal dropping method, the bonding process is carried out after the liquid crystal is dropped on the substrate. Hence, if a thermo-hardening sealant is used as the sealant 7, the sealant 7 flows out from the initial position during a heating process, thereby contaminating the liquid crystal 5. Therefore, an ultraviolet (UV)-hardening sealant is the choice of the sealant 7 in the liquid crystal dropping method.
Referring to FIG. 1D, UV rays from a UV emitting device 9 are irradiated on the entire surface of the attached substrates 1 and 3 including the sealant 7 to harden the sealant 7.
However, if the UV-rays are irradiated on an active area inside the sealant-formed area, device characteristics on the substrate such as thin film transistors and the like are degraded. Moreover, a pre-tilt angle of the alignment layer for the initial alignment of the liquid crystal is changed.